High-strength cold-rolled steel sheet having excellent surface whiteness, spot weldability, and chemical conversion treatability, and method for manufacturing same

Abstract

A cold-rolled steel sheet according to an embodiment of the present invention has a room-temperature tensile strength of 980 MPa or more, with the core portion thereof including 3 to 15% by area fraction of an austenite phase and other residual structures, and satisfies the following carbon concentration flattening index of equation (1) at a depth of 50 μm or more in a thickness direction from the outermost surface of the cold-rolled steel sheet, as analyzed by glow discharge spectrometry (GDS), and the following equation (1a-1) of 0.75 or less, the following equation (2a-1) of 0.80 or less, and the following equation (3a-1) of 0.85 or less. Equation (1): equation (1a-1): equation (2a-1): equation (3a-1): (wherein the core portion refers to a position at 1 / 4t in the thickness direction from the outermost surface of the cold-rolled steel sheet, Cx,GDS represents the carbon concentration at a depth x in the thickness direction, and d is a natural number of 40 to 100).

Description

High-strength cold-rolled steel sheet with excellent surface whiteness, spot weldability, and chemical treatment properties, and a method for manufacturing the same

[0001] The present invention relates to a steel sheet suitable for use as an automotive material, and more specifically, to a high-strength cold-rolled steel sheet having excellent surface whiteness, excellent spot weldability and chemical treatment properties, and a method for manufacturing the same.

[0002] Recently, the automotive industry has seen an increasing demand for vehicle body lightweighting to meet stricter environmental regulations and improve fuel efficiency. To address these demands, high-strength steel sheets are being applied as automotive materials to achieve lightweighting through reduced thickness while simultaneously enhancing safety. For this purpose, various types of high-strength steel, such as precipitation-strengthened, solid solution-strengthened, and phase-transformation-strengthened types, have been developed.

[0003] Among high-strength steels utilizing phase transformation, TRIP steel (Transformation Induced Plasticity Steel) in particular provides an excellent combination of strength and ductility by utilizing the transformation-induced plasticity phenomenon, in which retained austenite transforms into martensite during deformation, thereby providing additional ductility. To stabilize this retained austenite, alloying elements such as Mn, Si, and Al must be added, and these elements all have the characteristic of having a high affinity for oxidation.

[0004] However, these alloying elements diffuse into the steel sheet surface during the annealing process to form oxides. In particular, Si oxide forms as a continuous film on the steel sheet surface, hindering the formation of a phosphate film during subsequent phosphating treatments and thereby degrading the processability. Additionally, Mn oxide forms in the form of particles, causing discoloration on the steel sheet surface.

[0005] Various measures have been proposed to address these issues. The first proposed method involves pre-plating with Ni prior to annealing to suppress the surface diffusion of alloying elements. While this method effectively inhibits the surface diffusion of Mn, it has limitations in that it cannot sufficiently suppress the diffusion of Si. Therefore, although the problem of steel sheet discoloration can be resolved, ensuring chemical treatment performance remains challenging.

[0006] The second proposed method is an internal oxidation method that raises the dew point temperature of the annealing furnace to oxidize alloying elements in the surface layer rather than on the surface. Through this method, Si oxides are formed in a dispersed form rather than as a continuous film, thereby improving chemical treatment performance. However, this method has the disadvantage of failing to resolve the discoloration problem because it does not sufficiently suppress the surface diffusion of Mn and has limitations in reducing the overall amount of surface oxides.

[0007] The third proposed method is pickling, which directly removes surface oxides. Pickling in a hydrochloric acid bath removes surface Mn oxides, thereby restoring the surface color of discolored steel sheets, while simultaneously removing Si oxides ensures chemical treatment capability. However, this method presents a problem of reduced productivity, as it requires high-concentration acid baths, high temperatures, and prolonged treatment, particularly for the removal of Si oxides.

[0008] Meanwhile, special caution is required when spot welding steels containing retained austenite, such as TRIP steel. In the case of plated materials, there is a problem where cracks may occur in the material during spot welding when retained austenite is introduced to ensure high strength and formability. Particularly given that spot welding standards for automotive steel have recently become more stringent, there is an urgent need to develop technologies to prevent cracking during spot welding.

[0009] Furthermore, even for unplated cold-rolled materials, when butt-welding with a plated material, the plating layer of the target material is exposed to the surface of the cold-rolled material, which can cause Liquid Metal Embrittlement (LME). In particular, austenite grain boundaries are highly susceptible to LME, leading to serious problems where the steel plate fractures during spot welding. Therefore, it is very important to ensure spot weldability even for cold-rolled materials containing residual austenite.

[0010] High-dew-point annealing is proposed as the most effective method to solve these spot weldability problems. High-dew-point annealing not only alters surface oxidation behavior but also reduces the carbon content in the surface layer of the steel sheet, thereby decreasing the fraction of residual austenite in the surface layer. Through this, relatively soft decarburized ferrite is introduced, which can suppress crack formation by relieving the stress applied during spot welding through plastic deformation.

[0011] However, the methods proposed so far merely offer individual solutions for each problem, and a comprehensive solution capable of simultaneously addressing surface discoloration, chemical treatment properties, and spot weldability has yet to be presented. In particular, there is a need for the development of technology that can simultaneously improve spot weldability through high-dew-point annealing and enhance chemical treatment properties and whiteness through surface oxide control.

[0012] In order to solve the above-mentioned problem, one aspect of the present invention aims to provide a cold-rolled steel sheet having excellent spot weldability while maintaining high strength of 980 MPa or more.

[0013] In addition, another aspect of the present invention aims to provide a cold-rolled steel sheet with excellent chemical treatment properties so that a uniform phosphate film can be formed.

[0014] In addition, another aspect of the present invention aims to provide a cold-rolled steel sheet having excellent surface whiteness.

[0015] The problems of the present invention are not limited to those described above. The problems of the present invention can be understood from the entirety of this specification, and those skilled in the art will have no difficulty understanding additional problems of the present invention.

[0016] A cold-rolled steel sheet according to one embodiment of the present invention is a cold-rolled steel sheet having a room temperature tensile strength of 980 MPa or higher, wherein the core of the cold-rolled steel sheet comprises an austenite phase of 3 to 15% in area fraction and other remainder structures, and when analyzed by GDS (Glow Discharge Spectrometry), the cold-rolled steel sheet satisfies the following carbon concentration flattening index formula (1) at a depth of 50 μm or more in the thickness direction from the outermost surface of the cold-rolled steel sheet, and the cold-rolled steel sheet satisfies the following formula (1a-1) of 0.75 or less, the following formula (2a-1) of 0.80 or less, and the following formula (3a-1) of 0.85 or less.

[0017] Equation (1):

[0018] Equation (1a-1):

[0019] Equation (2a-1):

[0020] Equation (3a-1):

[0021] (Here, the core refers to the point 1 / 4t in the thickness direction from the outermost surface of the cold-rolled steel sheet, and C x,GDS represents the carbon concentration at depth x in the thickness direction, where d is a natural number between 40 and 100)

[0022] In addition, a cold-rolled steel sheet according to one embodiment of the present invention may satisfy the following formula (1b-1) being 0.55 or less, the following formula (2b-1) being 0.50 or less, and the following formula (3b-1) being 0.80 or less.

[0023] Equation (1b-1):

[0024] Equation (2b-1):

[0025] Equation (3b-1):

[0026] In addition, a cold-rolled steel sheet according to one embodiment of the present invention may satisfy the following formula (1c-1) at 0.25 or less, the following formula (2c-1) at 0.40 or less, and the following formula (3c-1) at 0.70 or less.

[0027] Equation (1c-1):

[0028] Equation (2c-1):

[0029] Equation (3c-1):

[0030] In addition, a cold-rolled steel sheet according to one embodiment of the present invention may satisfy the following formula (1a-2) at 0.85 or less, the following formula (2a-2) at 0.90 or less, and the following formula (3a-2) at 0.95 or less.

[0031] Equation (1a-2):

[0032] Equation (2a-2):

[0033] Equation (3a-2):

[0034] In addition, a cold-rolled steel sheet according to one embodiment of the present invention may satisfy the following formula (1b-2) being 0.70 or less, the following formula (2b-2) being 0.70 or less, and the following formula (3b-2) being 0.90 or less.

[0035] Equation (1b-2):

[0036] Equation (2b-2):

[0037] Equation (3b-2):

[0038] In addition, a cold-rolled steel sheet according to one embodiment of the present invention may satisfy the following formula (1c-2) at 0.35 or less, the following formula (2c-2) at 0.60 or less, and the following formula (3c-2) at 0.95 or less.

[0039] Equation (1c-2):

[0040] Equation (2c-2):

[0041] Equation (3c-2):

[0042] In addition, a cold-rolled steel sheet according to one embodiment of the present invention may satisfy the following formula (4) at 0.80 or less.

[0043] Equation (4):

[0044] (Here, Mn x,GDS represents the Mn concentration at depth x in the thickness direction, and Si x,GDS represents the Si concentration at depth x in the thickness direction, and (Mn+Si)0 represents the sum of the Mn and Si content in the core of the cold-rolled steel sheet based on OES analysis.

[0045] In addition, the cold-rolled steel sheet according to one embodiment of the present invention may have a concentration of (Mn+Si)O of 3.2 to 4.8 weight%.

[0046] In addition, the cold-rolled steel sheet according to one embodiment of the present invention may have a surface whiteness of 65 or higher.

[0047] In addition, a cold-rolled steel sheet according to one embodiment of the present invention may have a phosphate coverage of 85% or more.

[0048] In addition, the cold-rolled steel sheet according to one embodiment of the present invention may include, in the remainder structure, 60% or more of martensite and bainite and 40% or less of ferrite as an area fraction with respect to the total microstructure.

[0049] A welding steel plate according to another embodiment of the present invention may have a total thickness of 4.1 to 4.7 mm, including an upper plate and a lower plate.

[0050] In addition, the welding steel plate according to one embodiment of the present invention may have a total thickness of less than 4.1 to 4.3 mm, including the top plate and the bottom plate.

[0051] A welding steel plate according to another embodiment of the present invention may have a total thickness of less than 4.3 to 4.5 mm, including an upper plate and a lower plate.

[0052] A welding steel plate according to another embodiment of the present invention may have a total thickness of 4.5 to 4.7 mm, including an upper plate and a lower plate.

[0053] In addition, the welding steel plate according to one embodiment of the present invention may not crack during spot welding.

[0054] A method for manufacturing a cold-rolled steel sheet according to another embodiment of the present invention comprises, in the method for manufacturing a cold-rolled steel sheet, a step of manufacturing a steel material having a composition in weight% of C: 0.15 to 0.27%, Mn: 2.1 to 3.0%, Si: 1.3 to 2.1%, Cr: 0.1 to 0.5%, B: 1 to 30 ppm, and the remainder being Fe and unavoidable impurities; a step of reheating the steel material; a step of hot-rolling the reheated slab; a step of cooling at a cooling rate of 10℃ / s or more after hot-rolling and coiling at a range of 450 to 700℃; and a step of cold-rolling and annealing, wherein the annealing step is A c1 Among the ranges above the temperature, the region with a dew point of -10℃ or higher is 35% or more.

[0055] In addition, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention comprises, the annealing step is A c1 Among the ranges above the temperature, the region with a dew point of -10℃ or higher may be 50% or more.

[0056] In addition, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention comprises, the annealing step is A c1 Among the ranges above the temperature, the region with a dew point of -10℃ or higher may be 70% or more.

[0057] In addition, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention may further include a step of pickling after the annealing step under conditions of hydrochloric acid concentration of 5% or more, hydrochloric acid bath temperature of 50°C or more, and processing time of 5 seconds or more.

[0058] In addition, in the method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention, the surface whiteness can be controlled to 65 or higher during the pickling step.

[0059] In addition, in the method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention, the phosphate coverage can be controlled to 85% or more in the pickling step.

[0060] According to the present invention, a high-strength cold-rolled steel sheet with excellent spot weldability can be provided by precisely controlling the characteristics of the surface layer of the steel sheet. In particular, by reducing the residual austenite fraction through decarburization of the surface layer and introducing a soft ferrite layer, LME cracking that may occur during spot welding can be effectively prevented.

[0061] Furthermore, according to the present invention, excellent chemical treatment performance and surface whiteness can be simultaneously secured by effectively controlling surface oxides through a combination of high dew point annealing and appropriate pickling treatment. In particular, by suppressing the formation of Si oxide films and effectively removing Mn oxides, both chemical treatment performance and surface quality can be improved.

[0062] In addition, according to the present invention, while securing the above characteristics, it is possible to maintain a high strength of 980 MPa or higher, making it highly suitable for application as an automotive material. In particular, since high strength and excellent processability can be secured simultaneously, it is possible to simultaneously satisfy the conflicting demands of vehicle body weight reduction and safety improvement.

[0063] FIG. 1 is a diagram showing the microstructure of a cold-rolled steel sheet and the method of measuring equations at each depth according to Example 1 of the present invention.

[0064] FIG. 2(a) is a photograph showing the surface whiteness of a cold-rolled steel sheet according to a comparative example, and FIG. 2(b) is a photograph showing the surface whiteness of a cold-rolled steel sheet according to Example 1 of the present invention.

[0065] FIG. 3(a) is a photograph showing the phosphate coverage obtained by image analysis of the surface of a cold-rolled steel sheet according to a comparative example using SEM, and FIG. 3(b) is a photograph showing the phosphate coverage obtained by image analysis of the surface of a cold-rolled steel sheet according to Example 1 of the present invention using SEM.

[0066] Figure 4 is a figure showing the method of measuring the microstructure of a cold-rolled steel sheet and the equations at each depth according to Example 2 of the present invention.

[0067] FIG. 5(a) is a photograph showing the surface whiteness of a cold-rolled steel sheet according to a comparative example, and FIG. 5(b) is a photograph showing the surface whiteness of a cold-rolled steel sheet according to Example 2 of the present invention.

[0068] Figure 6(a) is a photograph showing the phosphate coverage obtained by image analysis of the surface of a cold-rolled steel sheet according to a comparative example using SEM, and Figure 6(b) is a photograph showing the phosphate coverage obtained by image analysis of the surface of a cold-rolled steel sheet according to Example 2 of the present invention using SEM.

[0069] FIG. 7 is a diagram showing the method of measuring the microstructure of a cold-rolled steel sheet and equations at each depth according to Example 3 of the present invention.

[0070] FIG. 8(a) is a photograph showing the surface whiteness of a cold-rolled steel sheet according to a comparative example, and FIG. 8(b) is a photograph showing the surface whiteness of a cold-rolled steel sheet according to Example 3 of the present invention.

[0071] FIG. 9(a) is a photograph showing the phosphate coverage obtained by image analysis of the surface of a cold-rolled steel sheet according to a comparative example using SEM, and FIG. 9(b) is a photograph showing the phosphate coverage obtained by image analysis of the surface of a cold-rolled steel sheet according to Example 3 of the present invention using SEM.

[0072] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the relevant technical field.

[0073] The terms used in this application are used merely to describe specific examples. For this reason, singular expressions include plural expressions unless the context clearly requires them to be singular. Additionally, it should be noted that terms such as “comprising” or “comprising” used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the existence of other features, steps, functions, components, or combinations thereof.

[0074] Meanwhile, unless otherwise defined, all terms used in this specification shall be understood to have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Accordingly, unless explicitly defined in this specification, specific terms should not be interpreted in an overly ideal or formal sense.

[0075] Additionally, terms such as "about," "substantially," etc., in this specification are used to mean at or near the stated value when inherent manufacturing and material tolerances are presented in the said sense, and are used to prevent unscrupulous infringers from unfairly exploiting the disclosed content in which precise or absolute values ​​are mentioned to aid in understanding the invention.

[0076] Unless otherwise specifically stated in this specification, the % indicating the content of each element is based on weight.

[0077] First, a cold-rolled steel sheet according to one aspect of the present invention will be described.

[0078] A cold-rolled steel sheet according to one embodiment of the present invention is a cold-rolled steel sheet having a room temperature tensile strength of 980 MPa or higher, wherein the core of the cold-rolled steel sheet comprises an austenite phase of 3 to 15% in area fraction and other remainder structures, and when analyzed by GDS (Glow Discharge Spectrometry), the cold-rolled steel sheet satisfies the following carbon concentration flattening index formula (1) at a depth of 50 μm or more in the thickness direction from the outermost surface of the cold-rolled steel sheet, and the cold-rolled steel sheet satisfies the following formula (1a-1) of 0.75 or less, the following formula (2a-1) of 0.80 or less, and the following formula (3a-1) of 0.85 or less.

[0079] Equation (1):

[0080] Equation (1a-1):

[0081] Equation (2a-1):

[0082] Equation (3a-1):

[0083] (Here, the core refers to the point 1 / 4t in the thickness direction from the outermost surface of the cold-rolled steel sheet, and C x,GDS represents the carbon concentration at depth x in the thickness direction, where d is a natural number between 40 and 100)

[0084] In the present invention, it is confirmed that the fundamental cause of the deterioration in spot weldability when welding a cold-rolled steel sheet with a galvanized steel sheet lies in the residual austenite fraction and the high hardness of the surface layer. Accordingly, it is confirmed that spot weldability can be improved by decarburizing the surface layer to lower the carbon solid solution content and reduce the austenite fraction, while simultaneously introducing soft ferrite.

[0085] Specifically, for a cold-rolled steel sheet with a room temperature tensile strength of 980 MPa or higher, the core may contain austenite in an area fraction of 3% to 15% as a microstructure and may contain other residual microstructures. Here, the core refers to the point 1 / 4t in the thickness direction from the outermost surface of the cold-rolled steel sheet. Generally, if the microstructure contains 3% or more austenite, LME cracking occurs during spot welding with a plated material, even in a cold-rolled steel sheet; the lower limit of the austenite fraction of 3% is intended to exclude the influence of a small amount of unintendedly formed austenite, and if the austenite phase fraction exceeds 15%, there is a problem of excessive LME cracking; therefore, the austenite phase fraction is controlled to 3% to 15%. The other residual microstructures of the above cold-rolled steel sheet may contain martensite and bainite in an area fraction of 60% or more and ferrite in an area fraction of 40% or less relative to the total microstructure. Martensite and bainite are the main structures for securing strength, and if their sum is less than 60%, it is difficult to achieve a target tensile strength of 980 MPa or higher. In addition, the cold-rolled steel sheet according to the present invention can effectively prevent LME cracking that may occur during spot welding by reducing the residual austenite fraction through decarburization of the surface layer and introducing a soft ferrite layer, and through such microstructure control, it is possible to secure high strength with a room temperature tensile strength of 980 MPa or higher while securing an elongation of 15% or higher.

[0086] Next, we would like to explain the control of surface layer characteristics, which is one of the most important features of the present invention. The surface layer characteristics must be controlled in stages according to the depth from the outermost surface of the cold-rolled steel sheet, and this is based on the following reasons.

[0087] First, the following characteristics are required at a depth of 10 µm from the surface. The average carbon concentration must be controlled to be 0.75 or less relative to the core. This is to reduce the residual austenite fraction through decarburization of the surface layer and to promote the formation of a soft ferrite layer. In fact, if the carbon content of the surface layer is higher than this, there is a problem that the possibility of LME cracking during spot welding increases significantly, so the following equation (1a-1) must satisfy 0.75 or less.

[0088] Equation (1a-1):

[0089] In addition, the austenite fraction must be controlled to be 0.80 or less relative to the core. This is intended to reduce LME sensitivity and improve crack resistance during spot welding. In particular, when spot welding with a plated material, it is important to minimize austenite grain boundaries in the region in direct contact with the plating layer, so the following equation (2a-1) must satisfy 0.80 or less.

[0090] Equation (2a-1):

[0091] The hardness must be controlled to be 0.85 or less relative to the core. This is to relieve the stress applied during spot welding through plastic deformation. If the hardness of the surface layer is higher than this, the possibility of cracking due to stress concentration increases, so the following equation (3a-1) must satisfy 0.85 or less.

[0092] Equation (3a-1):

[0093] Next, the following characteristics can be controlled at a depth of 20 µm from the surface. As the degree of decarburization gradually decreases from the surface to the depth, the values ​​of the ratio of average carbon concentration, the ratio of austenite phase fraction, and the ratio of hardness at a depth of 20 µm are higher than at the aforementioned depth of 10 µm.

[0094] Accordingly, the average carbon concentration at a depth of 20 μm in the thickness direction from the outermost surface of the cold-rolled steel sheet according to the present invention can be controlled to be 0.85 or less compared to the core.

[0095] If the carbon content of the surface layer is higher than 0.35, there is a problem that the possibility of LME cracking during spot welding increases significantly, so the following equation (1a-2) must satisfy 0.85 or less.

[0096] Equation (1a-2):

[0097] In addition, the austenite fraction must be controlled to be 0.90 or less relative to the core. This is intended to reduce LME sensitivity and improve crack resistance during spot welding. In particular, when spot welding with a plated material, it is important to minimize austenite grain boundaries in the region in direct contact with the plating layer, so the following equation (2a-2) must satisfy 0.90 or less.

[0098] Equation (2a-2):

[0099] The hardness must be controlled to be 0.95 or less relative to the core. This is to relieve the stress applied during spot welding through plastic deformation. If the hardness of the surface layer is higher than this, the possibility of cracking due to stress concentration increases, so the following equation (3a-2) must satisfy 0.95 or less.

[0100] Equation (3a-2):

[0101] In addition, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention comprises, the annealing step is A c1Among the ranges above the temperature, the region with a dew point of -10℃ or higher may be 50% or more.

[0102] First, the following characteristics are required at a depth of 10 µm from the surface. The average carbon concentration must be controlled to be 0.55 or less relative to the core. This is to reduce the residual austenite fraction through decarburization of the surface layer and to promote the formation of a soft ferrite layer. In fact, if the carbon content of the surface layer is higher than this, there is a problem that the possibility of LME cracking during spot welding increases significantly, so the following equation (1b-1) must satisfy 0.55 or less.

[0103] Equation (1b-1):

[0104] In addition, the austenite fraction must be controlled to be 0.50 or less relative to the core. This is intended to reduce LME sensitivity and improve crack resistance during spot welding. In particular, when spot welding with a plated material, it is important to minimize austenite grain boundaries in the region in direct contact with the plating layer, so the following equation (2b-1) must satisfy 0.50 or less.

[0105] Equation (2b-1):

[0106] The hardness must be controlled to be 0.80 or less relative to the core. This is to relieve the stress applied during spot welding through plastic deformation. If the hardness of the surface layer is higher than this, the possibility of cracking due to stress concentration increases, so the following equation (3b-1) must satisfy 0.80 or less.

[0107] Equation (3b-1):

[0108] Next, the following characteristics can be controlled at a depth of 20 µm from the surface. As the degree of decarburization gradually decreases from the surface to the depth, the values ​​of the ratio of average carbon concentration, the ratio of austenite phase fraction, and the ratio of hardness at a depth of 20 µm are higher than at the aforementioned depth of 10 µm.

[0109] Accordingly, the average carbon concentration at a depth of 20 μm in the thickness direction from the outermost surface of the cold-rolled steel sheet according to the present invention can be controlled to be 0.70 or less compared to the core.

[0110] If the carbon content of the surface layer is higher than 0.35, there is a problem that the possibility of LME cracking during spot welding increases significantly, so the following equation (1b-2) must satisfy 0.70 or less.

[0111] Equation (1b-2):

[0112] In addition, the austenite fraction must be controlled to be 0.70 or less relative to the core. This is intended to reduce LME sensitivity and improve crack resistance during spot welding. In particular, when spot welding with a plated material, it is important to minimize austenite grain boundaries in the region in direct contact with the plating layer, so the following equation (2b-2) must satisfy 0.70 or less.

[0113] Equation (2b-2):

[0114] The hardness must be controlled to be 0.90 or less relative to the core. This is to relieve the stress applied during spot welding through plastic deformation. If the hardness of the surface layer is higher than this, the possibility of cracking due to stress concentration increases, so the following equation (3b-2) must satisfy 0.90 or less.

[0115] Equation (3b-2):

[0116] In addition, a cold-rolled steel sheet according to one embodiment of the present invention may satisfy the following formula (1c-1) at 0.25 or less, the following formula (2c-1) at 0.40 or less, and the following formula (3c-1) at 0.70 or less.

[0117] Equation (1c-1):

[0118] Equation (2c-1):

[0119] Equation (3c-1):

[0120] First, the following characteristics are required at a depth of 10 µm from the surface. The average carbon concentration must be controlled to be 0.25 or less relative to the core. This is to reduce the residual austenite fraction through decarburization of the surface layer and to promote the formation of a soft ferrite layer. In fact, if the carbon content of the surface layer is higher than this, there is a problem that the possibility of LME cracking during spot welding increases significantly, so the following equation (1c-1) must satisfy 0.25 or less.

[0121] Equation (1c-1):

[0122] In addition, the austenite fraction must be controlled to be 0.40 or less relative to the core. This is intended to reduce LME sensitivity and improve crack resistance during spot welding. In particular, when spot welding with a plated material, it is important to minimize austenite grain boundaries in the region in direct contact with the plating layer, so the following equation (2c-1) must satisfy 0.40 or less.

[0123] Equation (2c-1):

[0124] The hardness must be controlled to be 0.70 or less relative to the core. This is to relieve the stress applied during spot welding through plastic deformation. If the hardness of the surface layer is higher than this, the possibility of cracking due to stress concentration increases, so the following equation (3c-1) must satisfy 0.70 or less.

[0125] Equation (3c-1):

[0126] Next, the following characteristics can be controlled at a depth of 25 µm from the surface. As the degree of decarburization gradually decreases from the surface to the depth, the values ​​of the ratio of average carbon concentration, the ratio of austenite phase fraction, and the ratio of hardness at a depth of 25 µm are higher than at the aforementioned depth of 10 µm.

[0127] Accordingly, the average carbon concentration at a depth of 25 μm in the thickness direction from the outermost surface of the cold-rolled steel sheet according to the present invention can be controlled to be 0.35 or less compared to the core.

[0128] If the carbon content of the surface layer is higher than 0.35, there is a problem that the possibility of LME cracking during spot welding increases significantly, so the following equation (1c-2) must satisfy 0.35 or less.

[0129] Equation (1c-2):

[0130] In addition, the austenite fraction must be controlled to be 0.60 or less relative to the core. This is intended to reduce LME sensitivity and improve crack resistance during spot welding. In particular, when spot welding with a plated material, it is important to minimize austenite grain boundaries in the region in direct contact with the plating layer, so the following equation (2c-2) must satisfy 0.60 or less.

[0131] Equation (2c-2):

[0132] The hardness must be controlled to be 0.95 or less relative to the core. This is to relieve the stress applied during spot welding through plastic deformation. If the hardness of the surface layer is higher than this, the possibility of cracking due to stress concentration increases, so the following equation (3c-2) must satisfy 0.95 or less.

[0133] Equation (3c-2):

[0134] In addition, the sum of the average concentrations of Mn and Si in the 0.05 μm region from the outermost surface of the cold-rolled steel sheet can be controlled to be 0.8 or less compared to the core. This is intended to suppress the formation of surface oxides and ensure chemical treatment performance and whiteness. Since both Mn and Si are elements with high oxidation affinity, if their surface concentrations are high, the formation of oxides becomes excessive, causing problems such as reduced chemical treatment performance and whiteness; therefore, the following equation (4) can satisfy 0.80 or less.

[0135] Equation (4):

[0136] (Here, Mn x,GDS represents the Mn concentration at depth x in the thickness direction, and Si x,GDS represents the Si concentration at depth x in the thickness direction, and (Mn+Si)0 represents the sum of the Mn and Si content in the core of the cold-rolled steel sheet based on OES analysis.

[0137] In addition, the cold-rolled steel sheet according to one embodiment of the present invention may have a concentration of (Mn+Si)O of 3.2 to 4.8 weight%. If the sum of the concentrations of Mn and Si in the core of the steel sheet is less than 3.2 weight%, it is difficult to secure the stability of the retained austenite, and the effect of suppressing carbide formation by Si is insufficient, making it difficult to secure more than 3% of retained austenite. Consequently, there is a problem in that the transformation-induced plasticity effect through martensitic transformation during deformation cannot be sufficiently obtained. If the sum of the concentrations of Mn and Si exceeds 4.8 weight%, a problem arises in which surface oxides are excessively formed. In particular, surface discoloration caused by Mn oxides becomes severe, and chemical treatment performance is significantly reduced by Si oxides. Excessive oxides make pickling difficult, leading to reduced productivity and increased costs. Furthermore, surface oxides degrade welding quality and cause increased susceptibility to LME. Accordingly, in one embodiment of the present invention, it is preferable to control the concentration of (Mn+Si)O in the cold-rolled steel sheet to 3.2 to 4.8 weight%, and accordingly, the surface whiteness may be 65 or higher and the phosphate coverage may be 85% or higher.

[0138] In addition, a welding steel plate according to another embodiment of the present invention is made of the cold-rolled steel plate described above, and the welding steel plate, including an upper plate and a lower plate, may have a total thickness of 4.1 to 4.7 mm, less than 4.1 to 4.3 mm, less than 4.3 to 4.5 mm, and 4.5 to 4.7 mm.

[0139] The welding steel plate according to the present invention is characterized by not cracking during spot welding.

[0140] The following describes a method for manufacturing a cold-rolled steel sheet according to one aspect of the present invention.

[0141] A method for manufacturing a cold-rolled steel sheet according to the present invention comprises the steps of: manufacturing a steel material having a composition in weight percent of C: 0.15 to 0.27%, Mn: 2.1 to 3.0%, Si: 1.3 to 2.1%, Cr: 0.1 to 0.5%, B: 1 to 30 ppm, and the remainder being Fe and unavoidable impurities; reheating the steel material; hot-rolling the reheated slab; cooling at a cooling rate of 10℃ / s or more after hot-rolling and coiling at a temperature in the range of 450 to 700℃; and cold-rolling and annealing, wherein the annealing step is A c1 Among the ranges above the temperature, the area with a dew point of -10℃ or higher must satisfy 35% or more, 50% or more, or 70% or more.

[0142] The steel may be in the form of a slab or an ingot, and the composition of the steel should be sufficient to control the microstructure of the cold-rolled steel sheet described above, for example, may include C: 0.15 to 0.27%, Mn: 2.1 to 3.0%, Si: 1.3 to 2.1%, Cr: 0.1 to 0.5%, B: 1 to 30 ppm, and the remainder being Fe and unavoidable impurities.

[0143] Carbon is the most important element for ensuring strength and forming retained austenite, and may be included in an amount of 0.15 to 0.27% to control the retained austenite fraction after annealing. Manganese is an austenite stabilizing element and is essential for the formation of retained austenite; it may be included in an amount of 2.1 to 3.0% to ensure the retained austenite fraction and surface whiteness. Silicon acts as a ferrite stabilizing element and simultaneously promotes the formation of retained austenite by suppressing carbide formation; it may be included in an amount of 1.3 to 2.1% to suppress the formation of surface oxides on the retained austenite fraction. Chromium is an element that improves hardenability and contributes to ensuring strength; it may be included in an amount of 0.1 to 0.5% for strength and strength assurance. Boron is an element that can significantly improve hardenability even with trace additions; it may be included in an amount of 1 to 30 ppm to control strength, weldability, and brittleness. The remainder is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during ordinary manufacturing processes, they cannot be excluded. As these impurities are known to any skilled person in ordinary manufacturing processes, all details thereof are not specifically mentioned in this specification.

[0144] A slab having the above composition is heated at 1100°C or higher. This is to ensure complete solid solution of alloying elements and a uniform austenite structure. The heating time may be maintained for at least 120 minutes so that the center of the slab can be sufficiently heated.

[0145] Hot rolling is performed at 900°C or higher. If the rolling start temperature is lower than this, it may be difficult to achieve uniform deformation, and the finish rolling temperature is maintained at 850°C or higher to ensure a uniform distribution of alloying elements.

[0146] In order to secure the microstructure described above and improve mechanical properties, the cooling rate after hot rolling is 10℃ / s or more, and the product can be coiled at 450~700℃.

[0147] Cold rolling is performed with a reduction rate of 40 to 70 percent. If the reduction rate is less than 40 percent, it is difficult to expect sufficient strength improvement, and if it exceeds 70 percent, problems may occur in subsequent processes due to excessive work hardening; therefore, it can be performed under normal temperature conditions with a reduction rate of 40 to 70 percent.

[0148] The annealing treatment, which is the core process of the present invention, is carried out as follows. The annealing temperature may be carried out under conventional cold rolling annealing temperature and time conditions, but the conditions are A in order to promote decarburization of the surface layer and control internal oxidation. c1 Among the ranges above the temperature, the area with a dew point of -10℃ or higher must satisfy 35% or more, 50% or more, or 70% or more. A c1 In the above range, if an atmosphere of -10°C or higher is formed at the dew point, carbon dissolved in the steel plate escapes by forming CO gas, resulting in a structure where the carbon concentration increases from the surface to the core. When the carbon solid solution content decreases, austenite is not formed during cooling, which reduces the residual austenite fraction and decreases the austenite grain boundaries, thereby improving spot weldability. Furthermore, as the formation of martensite and bainite is reduced and the main structure consists of ferrite, the surface layer hardness decreases and plastic deformation becomes favorable, allowing it to withstand the stress applied during spot welding and thus improving spot weldability.

[0149] Specifically, the following effects can be achieved depending on the ratio of the time during which the dew point is -10℃ or higher during the total time during which the steel plate temperature is above Ac1:

[0150] (1) When the ratio of time above the dew point of -10℃ is 35% or more:

[0151] At a depth of 10㎛ from the surface, the average carbon concentration can be achieved to be 0.75 or less relative to the core, the austenite fraction to be 0.80 or less relative to the core, and the hardness to be 0.85 or less relative to the core. Additionally, at a depth of 20㎛ from the surface, the average carbon concentration can be achieved to be 0.85 or less relative to the core, the austenite fraction to be 0.90 or less relative to the core, and the hardness to be 0.95 or less relative to the core.

[0152] These surface decarburization characteristics enable basic spot weldability, and in particular, sufficient decarburization in the 10㎛ surface area can suppress the occurrence of LME cracks. Additionally, when combined with pickling treatment, a surface whiteness of 65 or higher and a phosphate coverage of 85% or higher can be achieved.

[0153] (2) When the proportion of time above the dew point of -10℃ is 50% or more:

[0154] At a depth of 10㎛ from the surface, the average carbon concentration can be achieved to be 0.55 or less relative to the core, the austenite fraction to be 0.50 or less relative to the core, and the hardness to be 0.80 or less relative to the core. Additionally, at a depth of 20㎛ from the surface, the average carbon concentration can be achieved to be 0.70 or less relative to the core, the austenite fraction to be 0.70 or less relative to the core, and the hardness to be 0.90 or less relative to the core.

[0155] Improved spot weldability can be secured through these surface decarburization characteristics. In particular, decarburization proceeds further to a depth of 20 µm in the surface layer, thickening the soft ferrite layer in the surface layer, and consequently, LME crack resistance is significantly improved. The austenite fraction at a depth of 10 µm is reduced to less than 50% of that in the core, significantly reducing the possibility of crack propagation through austenite grain boundaries during spot welding.

[0156] (3) When the proportion of time above the dew point of -10℃ is 70% or more:

[0157] At a depth of 10 µm from the surface, the average carbon concentration can be achieved to be 0.25 or less relative to the core, the austenite fraction to be 0.40 or less relative to the core, and the hardness to be 0.70 or less relative to the core. Additionally, at a depth of 25 µm from the surface, the average carbon concentration can be achieved to be 0.35 or less relative to the core, the austenite fraction to be 0.60 or less relative to the core, and the hardness to be 0.95 or less relative to the core.

[0158] Optimal spot weldability can be secured through these surface decarburization characteristics. Sufficient decarburization occurs up to a depth of 25㎛ in the surface layer, forming a soft ferrite layer; in the 10㎛ surface region, the austenite fraction is significantly reduced to 40% or less of the core, and the carbon concentration is lowered to 25% or less of the core, so cracks hardly occur during spot welding. In particular, the hardness of the surface layer is reduced to 70% or less compared to the core, allowing the stress applied during spot welding to be effectively relieved through plastic deformation.

[0159] In addition, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention may further include a pickling step after the annealing step under conditions of a hydrochloric acid concentration of 5% or more, a hydrochloric acid bath temperature of 50°C or more, and a treatment time of 5 seconds or more. In the present invention, the amount of Mn and Si dissolved in the core is significantly reduced as surface oxides are removed by undergoing a pickling process in a hydrochloric acid bath after the annealing process. Since Mn oxides are removed, the discolored steel sheet recovers its whiteness to its original level, and since Si oxides are removed, the chemical treatment properties also become excellent. That is, in the pickling step, the surface whiteness of the cold-rolled steel sheet according to the present invention can be controlled to 65 or more and the phosphate coverage to 85% or more.

[0160] The present invention will be explained in more detail below through examples. However, these examples are intended to illustrate the present invention and the scope of the present invention is not limited to these examples.

[0161] (Example)

[0162] Example 1

[0163] A slab satisfying a composition having a tensile strength of 980 MPa or higher and containing, in weight percent, C: 0.15 to 0.27%, Mn: 2.1 to 3.0%, Si: 1.3 to 2.1%, Cr: 0.2 to 0.4%, B: 10 to 20 ppm, and the remainder being Fe and unavoidable impurities, was prepared and heated at 1200°C for 2 hours. Subsequently, hot rolling was performed at 900°C, followed by cooling at a rate of 10°C / s and coiling at 550°C. Afterward, cold rolling was performed with a reduction rate of 60%, annealing was performed at a temperature of 700°C or higher, and pickling was performed under conditions of a hydrochloric acid concentration of 5% or higher, a hydrochloric acid bath temperature of 50°C or higher, and a treatment time of 5 seconds or more to produce a cold-rolled steel sheet. Table 1 below shows the carbon concentration, austenite fraction, and hardness values ​​at the core (1 / 4 t point) of each cold-rolled steel sheet.

[0164] Classification CO(Mn+Si) 0γ 0H 0 Dew Point -10℃ or higher Ratio (%) Maximum Dew Point (℃) within Range Pickling Status Comparative Example 1 - 10.2 113.8 07 10.3% 55 8.5 325.2 ○ Comparative Example 2 - 10.2 28 3.9 111 11.1% 56 9.7 246.3 ○ Comparative Example 3 - 10.2 20 3.8 43 10.6% 57 3.4 334.7 ○ Comparative Example 4 - 10.2 0 14.1 12 13.8% 51 3.6 21 11.3 ○ Comparative Example 5 - 10.1 68 4.0 109.4% 48 7.1 19 10.4 ○ Comparative Example 6 - 10.1 78 4.2 10 14.2% 49 6.8 224.4 ○ Comparative Example 7-10.18 13.24 16.4% 53 3.17 16.9 △Comparative Example 8-10.18 13.34 17.8% 53 1.14 85.3 △Comparative Example 9-10.16 74.07 98.4% 47 6.85 18.7 XComparative Example 10-10.21 23.22 75.9% 49 8.65 213.5 XComparative Example 11-10.22 54.11 210.7% 51 2.32 615.2 △Comparative Example 12-10.23 73.58 911.1% 51 1.93 04.6 △Comparative Example 13-10.17 23.27 47.4% 49 5.71 38.9 XComparative Example 14-10.1933.6118.8%453.7296.1○Comparative Example 15-10.1843.5138.4%537.6583.2△Comparative Example 16-10.1794.0949.1%518.64810.7△Comparative Example 17-10.1893.36813.7%532.8479.9X Comparative Example 18-10.2474.13518.0%560.7618.7○Comparative Example 19-10.1424.2641.5%409.4178.8○Comparative Example 20-10.1814.8257.2%508.34210.2○Comparative Example 21-10.2074.0119.3%491.20-41.5○Invention Example 1-10.1923.9686.9%496.8628.2○Invention Example 2-10.1984.3818.4%500.7579.1○Invention Example 3-10.1744.1129.8%547.13716.7○Invention Example 4-10.2274.07910.8%492.5477.3○Invention Example 5-10.2193.6485.7%542.6448.5○Invention Example 6-10.1853.2237.1%523.7516.7○Invention Example 7-10.2193.71210.1%511.1599.5○

[0165] Figure 1 illustrates a method for measuring the thickness direction characteristics of a cold-rolled steel sheet according to Example 1 of the present invention, showing a method for measuring carbon concentration at different depths from the outermost surface through GDS analysis. It can be confirmed that the carbon concentration decreases due to decarburization in the surface layer, and at depths of 50 μm or more, it becomes flattened, approaching the carbon concentration of the core (1 / 4 t point). Equations (1a-1), (2a-1), and (3a-1) represent the ratios of carbon concentrations measured at depths of 10 μm, 20 μm, and 50 μm from the surface, respectively. Additionally, Tables 2 and 3 below show the average carbon concentration, austenite fraction, hardness fraction, and LME evaluated from the outermost surface of the cold-rolled steel sheet to depths of 10 μm and 20 μm in the thickness direction, as well as at depths of 10 μm and 20 μm.

[0166] The evaluation of various characteristics in the present invention was carried out in the following manner.

[0167] The C concentration (C0) in the core of the steel plate is measured through OES (Optical Emission Spectrometry) analysis, and the surface layer C concentration ratio (X1) is calculated as follows by measuring the elemental profile from the surface to a depth d of at least 50㎛ in the thickness direction using GDS (Glow Discharge Spectroscopy), and then denoting the concentration at which the C concentration (Cx,GDS) satisfies the flattening index equation (1) as C1 and the average C concentration up to a specific depth x as C2. From the equation below, the surface layer C concentration ratio (C2 / C1, x=10) from the surface layer to 10㎛ is given as equation (1a-1), and the surface layer C concentration ratio (C2 / C1, x=20) up to 20㎛ is given as equation (1a-2). The following equation (1) means that flattening has occurred when the difference between the average C concentration values ​​within two adjacent locations (d-2 to d, d-3 to d-1) is less than 0.3% of the average concentration value inside.

[0168] Equation (1):

[0169] ,

[0170] The austenite fraction (γ0) in the core of the steel plate is calculated by measuring EBSD in a 36㎛*36㎛ area at 1 / 4 of the thickness of each steel plate. The austenite fraction in the surface layer is calculated by measuring EBSD in a 12㎛ (x-6㎛ ~ x+6㎛)*36㎛ area with sufficient image quality at depth x, and then measuring the austenite fraction (γ1). The ratio of the austenite phase fraction (γ1 / γ0, x=10) at the 10㎛ point of the surface layer is given by Equation (2a-1), and the ratio of the austenite phase fraction (γ1 / γ0, x=20) at the 20㎛ point of the surface layer is given by Equation (2a-2).

[0171] The hardness (H0) of the steel plate was calculated by determining the average Hv value by measuring three times with a 3g load using a microhardness tester at a point 1 / 4 of the thickness. The hardness of the surface layer was determined by measuring three times with a 3g load at a depth x and determining the average Hv value (H x ) was calculated, and the hardness ratio (H at the 10㎛ point) x / H0, x=10) is given by Equation (3a-1), and the hardness ratio (H) at a point of 20㎛ in the surface layer x / H0, x=20) was given as Equation (3a-2).

[0172] The Mn+Si concentration in the core of the steel plate is measured through OES analysis (Mn+Si)0, and the Mn+Si concentration ratio equation (4) in the surface layer is obtained by measuring the elemental profile from the surface to a depth x of up to 2㎛ in the thickness direction using GDS (Mn x,GDS , Si x,GDS The surface layer Mn+Si concentration ratio is calculated using the following formula.

[0173] Equation (4):

[0174] The high dew point exposure ratio of the steel plate is determined by considering the volume inside the annealing furnace, measuring the dew point of gas extracted from at least 15 points—specifically the upper, middle, and lower layers in the vertical direction and at least 5 points in the horizontal direction—using a dew point meter, and then using this as the steel plate temperature A c1 It was converted into the ratio of the time during which the dew point exceeds -10℃ out of the total time. The maximum dew point of the atmosphere gas around the steel plate within each section is shown in Table 1.

[0175] The pickling bath treatment applied to the steel plate was judged as X if not performed, O if performed with a hydrochloric acid concentration of 5% or more, a hydrochloric acid bath temperature of 50℃ or more, and a treatment time of 5 seconds or more, and △ if any of the conditions among the hydrochloric acid concentration, hydrochloric acid bath temperature, and treatment time were insufficient.

[0176] Alloyed hot-dip galvanized steel plates with a tensile strength of 980 MPa or higher were processed into 4 cm x 13 cm sheets, stacked vertically to form a triple layer, and spot welding was performed with the electrode tilted at a 10º angle. The total thickness of the welding steel plates was selected to be 4.6 mm, and each plate used was at least 1.4 mm thick. Prior to the spot welding evaluation, all evaluations underwent at least 12 preliminary evaluations, and the Expulsion Current (I EXP We evaluate ), and this evaluation is I EXP -0.5kA, I EXP After evaluating the same material combination eight times with a maximum current of -1.0 Ka, the cross-sections of each evaluation specimen were subjected to electrical discharge machining and observed using an OM to determine whether C-type cracks had occurred. The spot welding current cycle, electrode type, and applied pressure were set under conditions where a typical developer could easily determine the pass / fail status of the spot welding. If no cracks occurred in any of the eight specimens, it was judged as LME OK, and if even one crack occurred, it was judged as NG.

[0177] For phosphate treatment, steel plates were processed into 8cm x 8cm pieces and treated with a degreasing solution for 120 seconds, a surface conditioning agent for 30 seconds, and a chemical treatment solution for 120 seconds. Afterward, the film weight was 1500–3000 mg / cm², and the surface layer was observed at 1,000x magnification to calculate the phosphate coverage area fraction.

[0178] The surface whiteness of the steel plates was measured using a spectrophotometer. After processing the specimens into 50mm x 50mm sizes, the color difference was measured at a total of three points per specimen. Subsequently, L (lightness), a (yellowness), and b (redness) were measured, and the Hunter whiteness (W) was calculated to determine the respective values. Here, Hunter whiteness is W = 100 - {(100-L) 2 +(a 2 +b 2} 1 / 2 It is calculated as.

[0179] Comparative Examples 1-1 to 6-1 showed LME defects because the region above the dew point of -10℃ was not 35%, so the reduction rate of C in the surface layer was insufficient (Comparative Examples 1-1, 2-1, 5-1, 6-1), the austenite fraction was high (Comparative Examples 3-1, 4-1, 5-1, 6-1), or the hardness fraction was high (Comparative Examples 2-1, 4-1, 5-1).

[0180] Comparative Examples 7-1 to 10-1 are results obtained where the dew point range of -10°C or higher was sufficiently secured, but pickling was not applied (Comparative Examples 9-1, 10-1), or the hydrochloric acid bath concentration, temperature, or pickling time was applied less (Comparative Examples 7-1, 8-1). As shown in Table 4 below, when looking at the sum of Mn+Si concentrations relative to the core, it can be confirmed that oxides are concentrated on the surface, showing a higher concentration than the core when pickling is not performed.

[0181] Comparative Examples 11-1 to 17-1 are cases where LME failure occurs when the high dew point range ratio is insufficient (Comparative Examples 11-1 to 14-1), cases where both whiteness and chemical treatment performance are defective when pickling is not applied (Comparative Examples 13-1, 17-1), and cases where whiteness or chemical treatment performance is defective when pickling is applied below a certain level (Comparative Examples 11-1, 12-1, 15-1, 16-1).

[0182] Comparative Example 21-1 is a comparative example in which high dew point annealing was not performed, and it was confirmed that LME defects occur when spot welding is performed with a plated material.

[0183] Classification Formula (1a-1)(%) Formula (2a-1)(%) Formula (3a-1)(%) LME Comparative Example 1 - 175.170.483.1NG Comparative Example 3 - 168.383.381.3NG Comparative Example 4 - 169.961.785.9NG Comparative Example 5 - 176.885.290.1NG Comparative Example 13 - 177.882.186.1NG Comparative Example 19 - 180.285.183.7OK Invention Example 1 - 150.542.671.5OK Invention Example 2 - 161.257.479.8OK Invention Example 3 - 174.472.480.1OK Invention Example 4 - 174.178.678.7OK Invention Example 5-171.865.884.1OK Invention Example 6-160.158.873.7OK Invention Example 7-158.752.871.5OK

[0184] Table 2 above shows the carbon average concentration ratio (1a-1), austenite fraction ratio (2a-1), and hardness ratio (3a-1) at a depth of 10 μm from the outermost surface of the cold-rolled steel sheet in the thickness direction for the core. Invention Examples 1-1 to 7-1, which satisfy Equation (1a-1), Equation (2a-1), and Equation (3a-1) according to the present invention, can be confirmed to have good LME evaluation results during spot welding.

[0185] On the other hand, in Comparative Examples 1-1, 3-1, 4-1, 5-1, and 13-1, it was confirmed that LME defects appeared when evaluating the plated material and spot welding because at least one of Equations (1a-1), (2a-1), and (3a-1) did not satisfy the scope of the present invention.

[0186] Comparative Example 19-1 is a cold-rolled steel sheet containing an austenite fraction of 3% or less, which is an example in which LME is good even when applied at a lower level than the high dew point range presented in the present invention.

[0187] Classification Formula (1a-2)(%) Formula (2a-2)(%) Formula (3a-2)(%) LME Comparative Example 2-186.371.495.9NG Comparative Example 4-178.295.191.1NG Comparative Example 6-182.492.988.7NG Comparative Example 11-182.781.397.1NG Comparative Example 12-185.787.696.2NG Comparative Example 13-187.287.788.5NG Comparative Example 14-183.793.197.2NG Comparative Example 18-165.866.782.6NG Comparative Example 19-186.792.097.6OK Invention Example 1-169.066.786.6OK Invention Example 2-166.879.484.1OK Invention Example 3-177.177.683.0OK Invention Example 4-181.685.394.0OK Invention Example 5-183.988.592.6OK Invention Example 6-172.373.985.1OK Invention Example 7-171.260.379.7OK

[0188] Table 3 above shows the carbon average concentration ratio (1a-2), the austenite fraction ratio (2a-2), and the hardness ratio (3a-2) at a depth of 20 μm from the outermost surface of the cold-rolled steel sheet in the thickness direction for the core. Invention Examples 1-1 to 7-1, which satisfy Equation (1a-2), Equation (2a-2), and Equation (3a-2) according to the present invention, can be confirmed to have good LME evaluation results during spot welding.

[0189] On the other hand, in Comparative Examples 2-1, 4-1, 6-1, 11-1, 12-1, 13-1, 14-1, and 19-1, it was confirmed that LME defects appeared when evaluating the plated material and spot welding because at least one of Equation (1a-2), Equation (2a-2), and Equation (3a-2) did not satisfy the scope of the present invention.

[0190] Comparative Example 18-1 is a cold-rolled steel sheet containing more than 15% austenite fraction, and is an example in which LME defects occurred during spot welding with a plated material even when the high dew point range presented in the present invention was sufficiently applied. The inherent spot weldability of the steel sheet is determined by the austenite fraction and the degree of austenite grain boundary distribution, and this aspect is not improved even when a strong level of decarburization is applied.

[0191] Classification Formula (4) (%) Whiteness Phosphate Coverage Comparative Example 7-11 1.46386 Comparative Example 8-11 32.87062 Comparative Example 9-12 55.45352 Comparative Example 10-11 99.24951 Comparative Example 11-187.66088 Comparative Example 12-11 01.37357 Comparative Example 13-11 27.36172 Comparative Example 15-193.25488 Comparative Example 16-11 27.36771 Comparative Example 17-13 01.54841 Comparative Example 20-198.86260 Invention Example 1-16 6.87191 Invention Example 2-164.27794 Invention Example 3-178.37292 Invention Example 4-151.86989 Invention Example 5-159.16891 Invention Example 6-168.272>95 Invention Example 7-161.278>95

[0192] FIG. 2(a) is a photograph showing the surface whiteness of a cold-rolled steel sheet according to a comparative example, and FIG. 2(b) is a photograph showing the surface whiteness of a cold-rolled steel sheet according to Example 1 of the present invention. In addition, FIG. 3(a) is a photograph showing the phosphate coverage obtained by image analysis of the surface of a cold-rolled steel sheet according to a comparative example using SEM, and FIG. 3(b) is a photograph showing the phosphate coverage obtained by image analysis of the surface of a cold-rolled steel sheet according to Example 1 of the present invention using SEM.

[0193] Table 4 above shows the formula (4) for the sum of the average concentrations of Mn and Si in the 0.05 μm region from the outermost surface of the cold-rolled steel sheet for the core.

[0194] According to Table 4 and Figures 2(a) and (b) above, it can be confirmed that Invention Examples 1-1 to 7-1 satisfying Formula (4) according to the present invention have a whiteness of 65 or higher and a phosphate coverage of 85 or higher when spot-coated.

[0195] On the other hand, according to Table 4 and Figures 3(a) and (b) above, Comparative Examples 7-1 to 13-1 and 15-1 to 17-1, which do not satisfy Formula (4), can be seen to have a whiteness of less than 65 and / or a phosphate coverage of less than 85.

[0196] Specifically, Comparative Examples 7-1 to 10-1 sufficiently secured a dew point range of -10°C or higher, but as a result of not applying pickling (Comparative Examples 9-1, 10-1) or applying a low concentration, temperature, or pickling time of the hydrochloric acid bath (Comparative Examples 7-1, 8-1), when looking at the sum of Mn+Si concentration relative to the core, it can be confirmed that oxides are concentrated on the surface, showing a higher concentration than the core when pickling is not performed. In Comparative Example 7-1, some surface Mn oxide remained, so the whiteness was not restored, and in Comparative Example 8-1, surface Si oxide was not completely removed, resulting in poor phosphating performance. Comparative Examples 9-1 and 10-1 are cases where pickling was not performed, and in this case, both whiteness and phosphating performance were poor.

[0197] Comparative Examples 11-1 to 17-1 can be seen as having LME defects when the high dew point range ratio is insufficient (Comparative Examples 11-1 to 14-1), having both whiteness and chemical treatment defects when pickling is not applied (Comparative Examples 13-1, 17-1), or having whiteness or chemical treatment defects when pickling is applied below a certain level (Comparative Examples 11-1, 12-1, 15-1, 16-1).

[0198] Comparative Example 20-1 is an example where the Mn and Si dissolved in the steel plate are above a certain level, and even when the pickling treatment presented in the present invention is applied, the removal of surface oxides is insufficient, resulting in poor whiteness and chemical treatment performance.

[0199] Example 2

[0200] A slab satisfying a composition having a tensile strength of 980 MPa or higher and containing, in weight percent, C: 0.15 to 0.27%, Mn: 2.1 to 3.0%, Si: 1.3 to 2.1%, Cr: 0.1 to 0.5%, B: 1 to 30 ppm, and the remainder being Fe and unavoidable impurities, was prepared and heated at 1200°C for 2 hours. Subsequently, hot rolling was performed at 900°C, followed by cooling at a rate of 10°C / s and coiling at 550°C. Afterward, cold rolling was performed with a reduction rate of 60%, annealing was performed at a temperature of 700°C or higher, and pickling was performed under conditions of a hydrochloric acid concentration of 5% or higher, a hydrochloric acid bath temperature of 50°C or higher, and a treatment time of 5 seconds or more to produce a cold-rolled steel sheet. Table 5 below shows the carbon concentration, austenite fraction, and hardness values ​​at the core (1 / 4 t point) of each cold-rolled steel sheet.

[0201] Classification CO(Mn+Si) 0γ 0H 0 Dew Point -10℃ or higher Ratio (%) Maximum Dew Point (℃) within Range Pickling Status Comparative Example 1 - 20.18 33.8 39 8.9% 51 0.8 43 8.1 ○ Comparative Example 2 - 20.19 53.8 03 9.7% 51 8.6 38 10.7 ○ Comparative Example 3 - 20.17 93.8 42 10.1% 49 8.1 41 9.3 ○ Comparative Example 4 - 20.18 63.9 41 11.5% 45 6.5 39 5.9 ○ Comparative Example 5 - 20.20 93.6 32 5.9% 46 3.2 34 8.8 ○ Comparative Example 6 - 20.21 13.5 99 8.1% 52 3.1 27 7.1 ○ Comparative Example 7-20.2143.7747.9%581.26310.1△Comparative Example 8-20.2094.2318.6%533.4521.7△Comparative Example 9-20.1833.5685.7%493.3573.8X Comparative Example 10-20.1893.6176.9%527.1595.9X Comparative Example 11-20.1814.1129.2%516.8286.1△Comparative Example 12-20.1514.1948.8%461.71913.5△Comparative Example 13-20.1993.86911.7%543.83715.2X Comparative Example 14-20.22 33.31 810.8%53 3.43 49.6○Comparative Example 15-20.20 13.64 98.9%50 7.16 79.1△Comparative Example 16-20.23 94.00 47.1%49 1.86 16.4△Comparative Example 17-20.17 23.93 96.2%48 2.16 16.3X Comparative Example 18-20.25 13.64 618.8%52 7.66 57.4○Comparative Example 19-20.14 74.01 71.3%41 1.92 18.6○Comparative Example 20-20.18 54.82 610.7%52 3.15 911.1○Comparative Example 21-20.18 23.8 167.9% 51 2.60 -42.7 ○ Invention Example 1-20.19 14.3 319.4% 57 9.2 8512.0 ○ Invention Example 2-20.21 74.0 310.8% 52 2.5 669.0 ○ Invention Example 3-20.19 23.21 28.6% 53 7.3 589.1 ○ Invention Example 4-20.22 13.5 217.8% 53 8.5 6815.8 ○ Invention Example 5-20.18 73.9 6813.7% 48 8.8 7113.2 ○ Invention Example 6-20.20 84.0 3712.1% 57 8.9 7011.6 ○ Invention Example 7-20.1963.7425.9%508.5524.6○

[0202] FIG. 4 illustrates a method for measuring the thickness direction characteristics of a cold-rolled steel sheet according to Example 2 of the present invention, showing a method for measuring carbon concentration at different depths from the outermost surface through GDS analysis. It can be confirmed that the carbon concentration decreases due to decarburization in the surface layer, and at depths greater than 50 μm, it becomes flattened to approach the carbon concentration of the core (1 / 4t point). Equations (1b-1), (2b-1), and (3b-1) represent the ratios of carbon concentrations measured at depths of 10 μm, 20 μm, and 50 μm from the surface, respectively. Additionally, Tables 6 and 7 below show the average carbon concentration, austenite fraction, hardness fraction, and LME evaluated from the outermost surface of the cold-rolled steel sheet to depths of 10 μm and 20 μm in the thickness direction, as well as at depths of 10 μm and 20 μm.

[0203] Comparative Examples 1-2 to 6-2 showed LME defects because the region above the dew point of -10℃ was not 50%, and as can be seen in Tables 6 and 7 below, the reduction rate of C in the surface layer was insufficient (Comparative Examples 1-2, 2-2, 4-2, 5-2), the austenite fraction was high (Comparative Examples 2-2, 3-2, 4-2, 6-2), or the hardness fraction was high (Comparative Examples 4-2, 5-2, 6-2).

[0204] Comparative Examples 7-2 to 10-2 are results obtained where the dew point range of -10°C or higher was sufficiently secured, but pickling was not applied (Comparative Examples 9-2, 10-2), or the hydrochloric acid bath concentration, temperature, or pickling time was applied less (Comparative Examples 7-2, 8-2). As shown in Table 8 below, when looking at the sum of Mn+Si concentrations relative to the core, it can be confirmed that oxides are concentrated on the surface, showing a higher concentration than the core when pickling is not performed.

[0205] Comparative Examples 11-2 to 17-2 are cases where LME failure occurred when the high dew point range ratio was insufficient (Comparative Examples 11-2 to 14-2), cases where both whiteness and chemical treatment were defective when pickling was not applied (Comparative Examples 13-2, 17-2), and cases where whiteness or chemical treatment was defective when pickling was applied below a certain level (Comparative Examples 11-2, 12-2, 15-2, 16-2).

[0206] Comparative Example 21-2 is a comparative example in which high dew point annealing was not performed, and it was confirmed that LME defects occur when spot welding is performed with a plated material.

[0207] Classification Formula (1b-1)(%) Formula (2b-1)(%) Formula (3b-1)(%) LME Comparative Example 1-257.248.975.7NG Comparative Example 2-246.851.274.1NG Comparative Example 5-256.642.782.7NG Comparative Example 6-252.154.681.1NG Comparative Example 11-257.259.379.2NG Comparative Example 12-259.143.676.2NG Comparative Example 13-252.353.782.9NG Comparative Example 14-252.340.686.7NG Comparative Example 18-228.632.767.2NG Comparative Example 19-261.253.382.5OK Invention Example 1-214.933.161.9OK Invention Example 2-238.136.971.5OK Invention Example 3-253.947.577.5OK Invention Example 4-237.823.966.8OK Invention Example 5-238.142.872.3OK Invention Example 6-221.638.973.1OK Invention Example 7-251.141.276.1OK

[0208] Table 6 above shows the carbon average concentration ratio (1b-1), austenite fraction ratio (2b-1), and hardness ratio (3b-1) at a depth of 10 μm from the outermost surface of the cold-rolled steel sheet in the thickness direction for the core. Inventive Examples 1-2 to 7-2, which satisfy Equation (1b-1), Equation (2b-1), and Equation (3b-1) according to the present invention, can be confirmed to have good LME evaluation results during spot welding. On the other hand, Comparative Examples 1-2, 2-2, 5-2, 6-2, 11-2 to 14-2, and 18-2 did not satisfy the scope of the present invention for any one or more of Equation (1b-1), Equation (2b-1), and Equation (3b-1), and it was confirmed that LME defects appeared during the plating and spot welding evaluation.

[0209] Comparative Example 19-2 is a cold-rolled steel sheet containing 3% or less of an austenite fraction, which is an example in which LME is good even when applied at a lower level than the high dew point range presented in the present invention.

[0210] Classification Formula (1b-2)(%) Formula (2b-2)(%) Formula (3b-2)(%) LME Comparative Example 1-269.365.885.1NG Comparative Example 2-272.768.381.8NG Comparative Example 3-260.773.885.6NG Comparative Example 4-275.176.693.9NG Comparative Example 12-272.372.791.4NG Comparative Example 14-268.381.295.6NG Comparative Example 18-241.750.776.8NG Comparative Example 19-272.381.294.7OK Invention Example 1-224.151.979.1OK Invention Example 2-247.164.786.7OK Invention Example 3-268.869.288.1OK Invention Example 4-246.231.6%78.8OK Invention Example 5-247.150.878.8OK Invention Example 6-226.361.181.7OK Invention Example 7-261.762.377.6OK

[0211] Table 7 above shows the carbon average concentration ratio (1b-2), austenite fraction ratio (2b-2), and hardness ratio (3b-2) from the outermost surface of the cold-rolled steel sheet to a depth of 20 μm in the thickness direction for the core. Invention Examples 1-2 to 7-2, which satisfy Equation (1b-2), Equation (2b-2), and Equation (3b-2) according to the present invention, can be confirmed to have good LME evaluation results during spot welding.

[0212] On the other hand, in Comparative Examples 1-2 to 4-2, 12-2, 14-2, and 19-2, at least one of Formula (1b-2), Formula (2b-2), and Formula (3b-2) did not satisfy the scope of the present invention, and it was confirmed that LME defects appeared when evaluating the plated material and spot welding.

[0213] Comparative Example 18-2 is a cold-rolled steel sheet containing more than 15% austenite fraction, and is an example in which LME defects occurred during spot welding with a plated material even when the high dew point range presented in the present invention was sufficiently applied. The inherent spot weldability of the steel sheet is determined by the austenite fraction and the degree of austenite grain boundary distribution, and this aspect is not improved even when a strong level of decarburization is applied.

[0214] Classification Formula (4) (%) Whiteness Phosphate Coverage Comparative Example 7-2132.85589 Comparative Example 8-298.56853 Comparative Example 9-2212.36180 Comparative Example 10-2189.74849 Comparative Example 11-2138.46189 Comparative Example 12-2132.27264 Comparative Example 13-2231.65553 Comparative Example 15-294.56388 Comparative Example 16-286.27055 Comparative Example 17-2167.85271 Comparative Example 20-291.85861 Invention Example 1-252.772>95 Invention Example 2-262.871>95 Invention Example 3-266.77791 Invention Example 4-263.772>95 Invention Example 5-276.27191 Invention Example 6-273.177>95 Invention Example 7-267.27088

[0215] FIG. 5(a) is a photograph showing the surface whiteness of a cold-rolled steel sheet according to a comparative example, and FIG. 5(b) is a photograph showing the surface whiteness of a cold-rolled steel sheet according to Example 2 of the present invention. In addition, FIG. 6(a) is a photograph showing the phosphate coverage obtained by image analysis of the surface of a cold-rolled steel sheet according to a comparative example using SEM, and FIG. 6(b) is a photograph showing the phosphate coverage obtained by image analysis of the surface of a cold-rolled steel sheet according to Example 2 of the present invention using SEM. Table 8 above shows the equation (4) of the sum of the average concentrations of Mn and Si in the 0.05 μm region from the outermost surface of the cold-rolled steel sheet relative to the core.

[0216] According to Table 8 and Figures 5(a) and (b) above, it can be confirmed that Invention Examples 1-2 to 7-2 satisfying Formula (4) according to the present invention have a whiteness of 65 or higher and a phosphate coverage of 85 or higher when spot-coated.

[0217] On the other hand, according to Table 8 and Figures 6(a) and (b) above, Comparative Examples 7-2 to 13-2 and 15-2 to 17-2, which do not satisfy Formula (4), can be seen to have a whiteness of less than 65 and / or a phosphate coverage of less than 85.

[0218] Specifically, Comparative Examples 7-2 to 10-2 sufficiently secured a dew point range of -10°C or higher, but were the result of not applying pickling (Comparative Examples 9-2, 10-2) or applying a low concentration, temperature, or pickling time of the hydrochloric acid bath (Comparative Examples 7-2, 8-2). When looking at the sum of Mn+Si concentration relative to the core, it can be confirmed that oxides are concentrated on the surface, showing a higher concentration than the core when pickling is not performed. In Comparative Example 7-2, some surface Mn oxide remained, so the whiteness was not restored, and in Comparative Example 8-2, surface Si oxide was not completely removed, resulting in poor phosphating performance. Comparative Examples 9-2 and 10-2 were cases where pickling was not performed, and in this case, both whiteness and phosphating performance were poor.

[0219] Comparative Examples 11-2 to 17-2 can be seen as having LME defects when the high dew point range ratio is insufficient (Comparative Examples 11-2 to 14-2), having both whiteness and chemical treatment defects when pickling is not applied (Comparative Examples 13-2, 17-2), or having whiteness or chemical treatment defects when pickling is applied below a certain level (Comparative Examples 11-2, 12-2, 15-2, 16-2).

[0220] Comparative Example 20-2 is an example where the Mn and Si dissolved in the steel plate are above a certain level, and even when the pickling treatment presented in the present invention is applied, the removal of surface oxides is insufficient, resulting in poor whiteness and chemical treatment performance.

[0221] Example 3

[0222] A slab satisfying a composition having a tensile strength of 980 MPa or higher and containing, in weight percent, C: 0.15 to 0.27%, Mn: 2.1 to 3.0%, Si: 1.3 to 2.1%, Cr: 0.1 to 0.5%, B: 1 to 30 ppm, and the remainder being Fe and unavoidable impurities, was prepared and heated at 1200°C for 2 hours. Subsequently, hot rolling was performed at 900°C, followed by cooling at a rate of 10°C / s and coiling at 550°C. Afterward, cold rolling was performed with a reduction rate of 60%, annealing was performed at a temperature of 700°C or higher, and pickling was performed under conditions of a hydrochloric acid concentration of 5% or higher, a hydrochloric acid bath temperature of 50°C or higher, and a treatment time of 5 seconds or more to produce a cold-rolled steel sheet. Table 9 below shows the carbon concentration, austenite fraction, and hardness values ​​at the core (1 / 4 t point) of each cold-rolled steel sheet.

[0223] Classification CO(Mn+Si) 0γ 0H 0 Dew Point -10℃ or higher Ratio (%) Maximum Dew Point (℃) within Range Pickling Status Comparative Example 1-30.19 23.34 88.8% 527.12 16.8○ Comparative Example 2-30.17 33.72 312.7% 498.62 613.5○ Comparative Example 3-30.18 13.67 87.1% 588.13 511.1○ Comparative Example 4-30.16 84.23 15.3% 513.35 113.2○ Comparative Example 5-30.22 94.10 36.9% 543.85 09.8○ Comparative Example 6-30.21 93.80 19.1% 486.84 815.5○ Comparative Example 7-30.1844.0418.7%531.18313.7△Comparative Example 8-30.1994.1327.6%573.28610.9△Comparative Example 9-30.1983.8995.8%503.1725.8X Comparative Example 10-30.1833.9016.3%537.17712.9X Comparative Example 11-30.1813.8418.9%496.84112.1△Comparative Example 12-30.1613.7717.3%512.1308.7△Comparative Example 13-30.1793.68911.1%538.5319.5X Comparative Example 14-30.21 13.88 113.2%47 2.94 59.9○Comparative Example 15-30.23 03.59 78.9%56 6.79 115.5△Comparative Example 16-30.22 94.01 06.8%57 1.18 816.1△Comparative Example 17-30.19 23.89 49.0%55 8.97 414.7X Comparative Example 18-30.24 13.98 917.3%55 0.38 813.7○Comparative Example 19-30.14 94.31 10.8%42 1.85 012.3○Comparative Example 20-30.17 24.80 711.8%51 1.98 113.2○Comparative Example 21-30.1793.7718.8%531.80-39.8○Invention Example 1-30.1834.32210.4%568.29521.4○Invention Example 2-30.2074.1126.8%512.48415.2○Invention Example 3-30.2193.9797.4%564.18217.7○Invention Example 4-30.2063.3899.3%493.79411.1○Invention Example 5-30.2313.46113.7%507.39213.5○Invention Example 6-30.1683.24211.2%523.17115.4○Invention Example 7-30.1863.8118.3%551.17613.6○

[0224] FIG. 7 illustrates a method for measuring the thickness direction characteristics of a cold-rolled steel sheet according to Example 3 of the present invention, showing a method for measuring carbon concentration at different depths from the outermost surface through GDS analysis. It can be observed that the carbon concentration decreases due to decarburization in the surface layer, and at depths greater than 50 μm, it becomes flattened to approach the carbon concentration of the core (1 / 4 t point). Equations (1c-1), (2c-1), and (3c-1) represent the ratios of carbon concentrations measured at depths of 10 μm, 25 μm, and 50 μm from the surface, respectively. Additionally, Tables 10 and 11 below show the average carbon concentration, austenite fraction, hardness fraction, and LME evaluated from the outermost surface of the cold-rolled steel sheet to depths of 10 μm and 25 μm in the thickness direction, as well as at depths of 10 μm and 25 μm.

[0225] Comparative Examples 1-3 to 6-3 showed LME defects because the region above the dew point of -10℃ was not 70%, and as can be seen in Tables 10 and 11 below, the reduction rate of C in the surface layer was insufficient (Comparative Examples 1-3, 2-3, 5-3, 6-3), the austenite fraction was high (Comparative Examples 2-3, 3-3, 5-3), or the hardness fraction was high (Comparative Examples 4-3, 5-3, 6-3).

[0226] Comparative Examples 7-3 to 10-3 are results obtained where the dew point range of -10°C or higher was sufficiently secured, but pickling was not applied (Comparative Examples 9-3, 10-3), or the hydrochloric acid bath concentration, temperature, or pickling time was applied less (Comparative Examples 7-3, 8-3). As shown in Table 12 below, when looking at the sum of Mn+Si concentrations relative to the core, it can be confirmed that oxides are concentrated on the surface, showing a higher concentration than the core when pickling is not performed.

[0227] Comparative Examples 11-3 to 17-3 are cases where LME failure occurred when the high dew point range ratio was insufficient (Comparative Examples 11-3 to 14-3), cases where both whiteness and chemical treatment were defective when pickling was not applied (Comparative Examples 13-3, 17-3), and cases where whiteness or chemical treatment was defective when pickling was applied below a certain level (Comparative Examples 11-3, 12-3, 15-3, 16-3).

[0228] Comparative Example 21-3 is a comparative example in which high dew point annealing was not performed, and it was confirmed that LME defects occur when spot welding with plated material.

[0229] Classification Formula (1c-1)(%) Formula (2c-1)(%) Formula (3c-1)(%) LME Comparative Example 1-327.238.969.8NG Comparative Example 3-321.241.765.5NG Comparative Example 4-322.938.771.7NG Comparative Example 11-328.142.168.7NG Comparative Example 13-323.143.172.8NG Comparative Example 14-325.931.574.2NG Comparative Example 19-329.162.571.3OK Invention Example 1-37.810.354.4OK Invention Example 2-310.025.759.1OK Invention Example 3-312.834.362.6OK Invention Example 4-39.633.353.8OK Invention Example 5-39.221.260.2OK Invention Example 6-317.830.761.9OK Invention Example 7-314.831.162.8OK

[0230] Table 10 above shows the carbon average concentration ratio (1c-1), austenite fraction ratio (2c-1), and hardness ratio (3c-1) from the outermost surface of the cold-rolled steel sheet to a depth of 10 μm in the thickness direction for the core. Invention Examples 1-3 to 7-3, which satisfy Equation (1c-1), Equation (2c-1), and Equation (3c-1) according to the present invention, can be confirmed to be good based on LME evaluation results during spot welding.

[0231] On the other hand, in Comparative Examples 1-3, 3-3, 4-3, 11-3, 13-3, and 14-3, it was confirmed that LME defects appeared when evaluating the plated material and spot welding because at least one of Equation (1c-1), Equation (2c-1), and Equation (3c-1) did not satisfy the scope of the present invention.

[0232] Comparative Example 19-3 is a cold-rolled steel sheet containing 3% or less of an austenite fraction, which is an example in which LME is good even when applied at a lower level than the high dew point range presented in the present invention.

[0233] Classification Formula (1c-2)(%) Formula (2c-2)(%) Formula (3c-2)(%) LME Comparative Example 2-341.764.979.8NG Comparative Example 5-340.173.295.5NG Comparative Example 6-338.958.796.1NG Comparative Example 12-336.761.796.7NG Comparative Example 18-330.758.791.1NG Invention Example 1-315.826.967.6OK Invention Example 2-317.441.189.4OK Invention Example 3-317.050.074.1OK Invention Example 4-323.340.068.8OK Invention Example 5-321.241.772.7OK Invention Example 6-323.253.773.9OK Invention Example 7-330.043.778.1OK

[0234] Table 11 above shows the carbon average concentration ratio (1c-2), austenite fraction ratio (2c-2), and hardness ratio (3c-2) from the outermost surface of the cold-rolled steel sheet to a depth of 25 μm in the thickness direction for the core. Invention Examples 1-3 to 7-3, which satisfy Equation (1c-2), Equation (2c-2), and Equation (3c-2) according to the present invention, can be confirmed to have good LME evaluation results during spot welding.

[0235] On the other hand, in Comparative Examples 2-3, 5-3, 6-3, and 12-3, at least one of Equation (1c-2), Equation (2c-2), and Equation (3c-2) did not satisfy the scope of the present invention, and it was confirmed that LME defects appeared when evaluating the plated material and spot welding.

[0236] Comparative Example 18-3 is a cold-rolled steel sheet containing an austenite fraction of 15% or more, which is an example in which LME defects occurred during spot welding with a plated material even when the high dew point range presented in the present invention was sufficiently applied. The inherent spot weldability of the steel sheet is determined by the austenite fraction and the degree of austenite grain boundary distribution, and this aspect is not improved even when a strong level of decarburization is applied.

[0237] Classification Formula (4) (%) Whiteness Phosphate Coverage (%) Comparative Example 7-310 1.35989 Comparative Example 8-39 9.76951 Comparative Example 9-328 2.34944 Comparative Example 10-315 9.86279 Comparative Example 11-312 1.85893 Comparative Example 12-39 0.17261 Comparative Example 15-313 1.76191 Comparative Example 16-315 0.16844 Comparative Example 17-322 1.34947 Comparative Example 18-37 3.471 >95 Comparative Example 20-311 2.75951 Invention Example 1-35 1.773 >95 Invention Example 2-35 5.876 >95 Invention Example 3-371.27293 Invention Example 4-375.773>95 Invention Example 5-379.17292 Invention Example 6-365.37694 Invention Example 7-363.77293

[0238] FIG. 8(a) is a photograph showing the surface whiteness of a cold-rolled steel sheet according to a comparative example, and FIG. 8(b) is a photograph showing the surface whiteness of a cold-rolled steel sheet according to Example 3 of the present invention. In addition, FIG. 9(a) is a photograph showing the phosphate coverage obtained by image analysis of the surface of a cold-rolled steel sheet according to a comparative example using SEM, and FIG. 9(b) is a photograph showing the phosphate coverage obtained by image analysis of the surface of a cold-rolled steel sheet according to Example 3 of the present invention using SEM. Table 12 above shows the equation (4) of the sum of the average concentrations of Mn and Si in the 0.05 μm region from the outermost surface of the cold-rolled steel sheet relative to the core.

[0239] According to Table 12 and Figures 8(a) and (b) above, it can be confirmed that Invention Examples 1-3 to 7-3 satisfying Formula (4) according to the present invention have a whiteness of 65 or higher and a phosphate coverage of 85 or higher when spot-coated.

[0240] On the other hand, according to Table 12 and Figures 9(a) and (b) above, Comparative Examples 7-3 to 12-3 and 15-3 to 18-3, which do not satisfy Formula (4), can be seen to have a whiteness of less than 65 and / or a phosphate coverage of less than 85.

[0241] Specifically, Comparative Examples 7-3 to 10-3 sufficiently secured a dew point range of -10°C or higher, but were the result of not applying pickling (Comparative Examples 9-3, 10-3) or applying a low concentration and temperature of the hydrochloric acid bath or a low pickling time (Comparative Examples 7-3, 8-3). When looking at the sum of Mn+Si concentrations relative to the core, it can be confirmed that oxides are concentrated on the surface, showing a higher concentration than the core when pickling is not performed. In Comparative Example 7-3, some surface Mn oxides remained, so the whiteness was not restored, and in Comparative Example 8-3, surface Si oxides were not completely removed, resulting in poor phosphating performance. Comparative Examples 9-3 and 10-3 were cases where pickling was not performed, and in this case, both whiteness and phosphating performance were poor.

[0242] Comparative Examples 11-3 to 17-3 show that LME failure occurs when the high dew point range ratio is insufficient (Comparative Examples 11-3 to 14-3), both whiteness and chemical treatment failure occur when pickling is not applied (Comparative Examples 13-3, 17-3), or whiteness or chemical treatment failure occurs when pickling is applied below a certain level (Comparative Examples 11-3, 12-3, 15-3, 16-3).

[0243] Comparative Example 20-3 is an example where the Mn and Si dissolved in the steel plate are above a certain level, and even when the pickling treatment presented in the present invention is applied, the removal of surface oxides is insufficient, resulting in poor whiteness and chemical treatment performance.

[0244] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible within the scope and concept of the claims set forth below.

Claims

In a cold-rolled steel sheet having a room temperature tensile strength of 980 MPa or higher, The core of the above cold-rolled steel sheet comprises an austenite phase of 3 to 15% in area fraction and other remainder structures, and When the above cold-rolled steel sheet is analyzed by GDS (Glow Discharge Spectrometry), it satisfies the following carbon concentration flattening index formula (1) at a depth of 50㎛ or more in the thickness direction from the outermost surface of the above cold-rolled steel sheet, and The above cold-rolled steel sheet satisfies the following formula (1a-1) being 0.75 or less, the following formula (2a-1) being 0.80 or less, and the following formula (3a-1) being 0.85 or less. Equation (1): Equation (1a-1): Equation (2a-1): Equation (3a-1): (Here, the core refers to the point 1 / 4t in the thickness direction from the outermost surface of the cold-rolled steel sheet, and C x,GDS represents the carbon concentration at depth x in the thickness direction, where d is a natural number between 40 and 100) In claim 1, The above cold-rolled steel sheet satisfies the following formula (1b-1) being 0.55 or less, the following formula (2b-1) being 0.50 or less, and the following formula (3b-1) being 0.80 or less. Equation (1b-1): Equation (2b-1): Equation (3b-1): In claim 1, The above cold-rolled steel sheet satisfies the following formula (1c-1) being 0.25 or less, the following formula (2c-1) being 0.40 or less, and the following formula (3c-1) being 0.70 or less. Equation (1c-1): Equation (2c-1): Equation (3c-1): In claim 1, The above cold-rolled steel sheet satisfies the following formula (1a-2) being 0.85 or less, the following formula (2a-2) being 0.90 or less, and the following formula (3a-2) being 0.95 or less. Equation (1a-2): Equation (2a-2): Equation (3a-2): In claim 2, The above cold-rolled steel sheet satisfies the following formula (1b-2) being 0.70 or less, the following formula (2b-2) being 0.70 or less, and the following formula (3b-2) being 0.90 or less. Equation (1b-2): Equation (2b-2): Equation (3b-2): In claim 3, The above cold-rolled steel sheet satisfies the following formula (1c-2) being 0.35 or less, the following formula (2c-2) being 0.60 or less, and the following formula (3c-2) being 0.95 or less. Equation (1c-2): Equation (2c-2): Equation (3c-2): In any one of claims 1 to 6, The above cold-rolled steel sheet is a cold-rolled steel sheet that satisfies the following formula (4) being 0.80 or less. Equation (4): (Here, Mn x,GDS represents the Mn concentration at depth x in the thickness direction, and Si x,GDS represents the Si concentration at depth x in the thickness direction, and (Mn+Si)0 represents the sum of the Mn and Si content in the core of the cold-rolled steel sheet based on OES analysis. In claim 7, A cold-rolled steel sheet having a concentration of (Mn+Si)O of 3.2 to 4.8 weight%. In claim 7, The above cold-rolled steel sheet is characterized by having a surface whiteness of 65 or higher. In claim 7, The above cold-rolled steel sheet is characterized by having a phosphate coverage of 85% or more. In any one of claims 1 to 3, A cold-rolled steel sheet in which the above residual structure comprises, in terms of area fraction with respect to the total microstructure, 60% or more of martensite and bainite and 40% or less of ferrite. A welding steel sheet comprising a cold-rolled steel sheet according to any one of claims 1 to 3, The above welding steel plate is a welding steel plate having a total thickness of less than 4.1 to 4.3 mm, including the top plate and the bottom plate. In claim 12, The above welding steel plate is characterized by not cracking during spot welding. In the method for manufacturing a cold-rolled steel sheet according to claim 1, A step of manufacturing steel comprising, in weight percent, C: 0.15 to 0.27%, Mn: 2.1 to 3.0%, Si: 1.3 to 2.1%, Cr: 0.1 to 0.5%, B: 1 to 30 ppm, and the remainder being Fe and unavoidable impurities; Step of reheating the above steel material; A step of hot rolling the above-mentioned reheated slab; A step of cooling at a cooling rate of 10℃ / s or more after hot rolling and coiling in the range of 450~700℃; and It includes the steps of cold rolling and annealing, The above annealing step is A c1 A method for manufacturing cold-rolled steel sheets, wherein the region with a dew point of -10℃ or higher among the temperature ranges is 35% or more. In the method for manufacturing a cold-rolled steel sheet according to claim 2, A step of manufacturing steel comprising, in weight percent, C: 0.15 to 0.27%, Mn: 2.1 to 3.0%, Si: 1.3 to 2.1%, Cr: 0.1 to 0.5%, B: 1 to 30 ppm, and the remainder being Fe and unavoidable impurities; Step of reheating the above steel material; A step of hot rolling the above-mentioned reheated slab; A step of cooling at a cooling rate of 10℃ / s or more after hot rolling and coiling in the range of 450~700℃; and It includes the steps of cold rolling and annealing, The above annealing step is A c1 A method for manufacturing cold-rolled steel sheets, wherein the region with a dew point of -10℃ or higher accounts for 50% or more of the range above the temperature. In the method for manufacturing a cold-rolled steel sheet of claim 3, A step of manufacturing steel comprising, in weight percent, C: 0.15 to 0.27%, Mn: 2.1 to 3.0%, Si: 1.3 to 2.1%, Cr: 0.1 to 0.5%, B: 1 to 30 ppm, and the remainder being Fe and unavoidable impurities; Step of reheating the above steel material; A step of hot rolling the above-mentioned reheated slab; A step of cooling at a cooling rate of 10℃ / s or more after hot rolling and coiling in the range of 450~700℃; and It includes the steps of cold rolling and annealing, The above annealing step is A c1 A method for manufacturing cold-rolled steel sheets, wherein the region with a dew point of -10℃ or higher accounts for 70% or more of the temperature range. In any one of claims 14 to 16, A method for manufacturing a cold-rolled steel sheet, further comprising a pickling step after the annealing step under conditions of hydrochloric acid concentration of 5% or more, hydrochloric acid bath temperature of 50℃ or more, and processing time of 5 seconds or more. In claim 17, A method for manufacturing a cold-rolled steel sheet, characterized in that the surface whiteness is controlled to 65 or higher during the above-mentioned pickling step. In claim 17, A method for manufacturing a cold-rolled steel sheet, characterized in that the phosphate coverage is controlled to 85% or more in the above-mentioned pickling step.

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